INTRODUCTION. 63 



tion to the size of the body, than in any other tribe 

 of insects. The forms which they assume are very 

 various ; but the most ordinary shape of the upper 

 pair is triangular, with the apex of the triangle to- 

 wards the body, while the outline of the under wings 

 approaches to circular. They are traversed by nume- 

 rous nervures, which give a great degree of strength 

 to the wing, and hold in tension the thin elastic 

 membrane of which it is composed. These nervures 

 are tubular, and are permeated by an aerial and 

 aqueous fluid, the action of which expands the wing 

 when in a moist and corrugated state after the in- 

 sect has emerged from the pupa. The principal 

 branches rise from the point where the wing is at- 

 tached to the body, and they divide towards the 

 other extremity into numerous ramifications. The 

 spaces into which the wing is divided by these ner- 

 vures, are denominated areolets by Kirby and Spence ; 

 and these authors regard the upper wings as divisible 

 into three larger longitudinal sections, which they 

 term areas. The costal area occupies the anterior 

 margin : the anal area, a narrow space along the 

 posterior margin ; and the intermediate area, all that 

 part of the wing lying between the two others. The 

 most conspicuous areolet in butterflies is towards 

 the centre of the wing, at the base, and is usually 

 closed on its outer side by transverse nervures. In 

 many instances, however, there are no transverse 

 nervures, and all the areolets are open towards the 

 outer side. 



